The present invention relates in general to an electronic gain control circuit and, more particularly, to a variable gain control circuit which includes a voltage reference circuit coupled to an input stage, another voltage reference circuit means coupled to an output stage, and an operational amplifier (op amp) responsive to the input signal for developing a dynamic control voltage to the input and output stages. The circuit permits the magnitude of the input signal and the output signal to exceed independent current sources within the respective voltage reference circuits.
Variable gain control circuits are used in a myriad of applications, including receivers, transmitters and high fidelity electronics, anywhere it is necessary to adjust the amplitude of a signal. Specific circuits include attenuators, volume controls, tone controls and audio mixers. A typical gain control circuit includes an input stage and an output stage connected through a common node to the output of an op amp which in turn controls the current in each of the stages. The op amp senses a particular voltage, set by the magnitude of input current, and adjusts its output voltage to compel the input stage to sink the input current. The same op amp output voltage is applied to the output stage and controls the magnitude of the output current. Thus, a particular input current results in an particular output current proportional to the ratio of the current sources within the respective input and output stages. The input and output terminals couple to current sources within the respective stages in such manner that the magnitude of the input and output currents is limited to that of the current source. Equivalently, these current sources must be sufficiently large as to handle the specifications of the input and output signals and, therefore can be unacceptable in battery applications where power consumption is an issue. Also, in high fidelity and high power applications the excessive current in the variable gain control circuit will generate noise through common impedances to other components degrading their performance.
Another gain control circuit, disclosed in U.S. Pat. No. 4225794, receives a bipolar signal, splits the positive and negative cycles, and processes the separate signals through unipolar log/anti-log amplifiers before recombining into a single bipolar format. Although this circuit may work well it can be susceptible to crossover distortion during the recombination process. Still another gain control circuit uses an input and output stage such that each stage includes a PNP transistor coupled in series with a NPN transistor operated as current sources and having an output at the interconnection of the transistors which amplifies any mismatch between the two current sources. This technique is undesirable due to the difficulty in matching the PNP and NPN transistors.
Hence, there exists a need for an improved variable gain control circuit which is not susceptible to the crossover distortion nor requires PNP to NPN transistor matching. In addition, the input and output signals should not be limited by the magnitude of current source references, indeed, the current sources should be substantially smaller than the maximum input and output signals to minimize power consumption and system noise generation.